Redshift--Independent Distances of Spiral Galaxies: \\ II. Internal Extinction at I Band
Session 72 -- High Z Line Measurements; Gravitational Microlensing: Distance Determination
Display presentation, Friday, January 14, 9:30-6:45, Salons I/II Room (Crystal Gateway)

## [72.12] Redshift--Independent Distances of Spiral Galaxies: \\ II. Internal Extinction at I Band

R. Giovanelli, M.P. Haynes (Cornell), J.J. Salzer (Wesleyan), G. Wegner (Dartmouth), L.N. DaCosta (Obs.Nac.--Brasil), W. Freudling (ESO/ST--ECF), P. Chamaraux (Meudon)

We analyze the photometric properties of a sample of 1450 Sbc--Sc galaxies with known redshifts, single--dish HI profiles and CCD I band images to derive laws that relate the measured isophotal radius at $\mu_I=23.5$, magnitude, scale length and HI flux to the face--on aspect. Our results show that the central regions of spiral galaxies are substantially less transparent than most previous determinations suggest, but not as opaque as claimed by Valentijn (1990). Regions in the disk farther than two or three scale lengths from the center are close to completely transparent. In addition to statistically derived relations for the inclination dependence of photometric parameters, we present the results of a modelling exercise that utilizes the triplex'' model of Disney et al. (1989). Within the framework of that model, late spiral disks at I band have central optical depths on the order of $\tau_I \sim$ 5 and dust absorbing layers with scale heights on the order of half that of the stellar component. We discuss our results in light of previous determinations of internal extinction relations and point out the substantial impact of internal extinction on the scatter of the Tully--Fisher relation. We also find that the visual diameters by which large catalogs are constructed (UGC, ESO--Uppsala) are nearly proportional to face--on isophotal diameters.